This invention relates to plasma arc apparatus and is particularly concerned with a construction for use with a plasma arc torch.
In plasma arc apparatus it is known to provide a torch, there being provided within the torch an electrode suitably connected to a source of supply, and there being the provision of the passage of gas through the torch and through a nozzle on the torch and for the supply of coolant to reduce the temperature of the electrode and the nozzle during use.
Thus, when the torch is started, and electrical power supplied to the electrode, an arc is struck between the electrode and the nozzle this causing ionization of the gas passing through the nozzle, producing a short jet of conductive plasma. That short jet of conductive plasma can be brought into close proximity with a workpiece to effect the work required, or, with the workpiece itself connected in the circuit, the workpiece then serves as an electrode for an arc that can be struck between the electrode and the workpiece itself.
By providing coolant to the torch there is the belief that there is the prevention of ionization of gas near the sides of the nozzle thereby leaving a constricted conductive path in the centre of the gas flow. This has the advantageous effect of concentrating the energy in the main arc into a narrow region, and by changing the electrical properties of the arc (e.g., by increasing the number of volts per length) enables more energy to be put into this region for a given arc current. The effect of this is to produce arc temperatures very mnuch higher than those in so-called free arcs and very high energy densities in the arc itself.
When such apparatus is used in welding, cutting or similar processes, energy can be applied very precisely to the workpiece where it is required to melt the workpiece, with very low heat dissipation into the surrounding material.
Hitherto there have been various constructions of torch intended to provide for the reasonably ready replacement of the electrode itself which is of course consumed. Thus, there is one known construction where the electrode is screwed on to an electrode tube lying centrally of the torch but here sealing of the electrode to the tube relies on the metal to metal contact between the electrode and the tube and can be somewhat unreliable. In addition to this even with the provision of coolant the electrode tube and electrode are inevitably heated and this can cause the electrode to become very tightly secured to the tube and hence increase the difficulty of removing a burnt out electrode. In an attempt to overcome this problem it is known in a screwed construction to provide additional sealing O-rings, and whilst this can solve the problem of effecting sealing still leaves the problem of electrode removal.
A construction of plasma arc apparatus with a torch in which the electrode can be removed and replaced with relative ease and which at the same time provides for effective sealing between the electrode and its support member is described and claimed in British Pat. No. 2095520B, and it is the object of the present invention to provide still further improvements to the plasma arc apparatus therein disclosed.